This paper presents a finite element simulation by COMSOL Multiphysics package to investigate the temperature distribution inside three-phase, three-core, 33 kV underground power cables (UGC) through a coupled electromagnetic-thermal modelling. The simulations are very controlled and fine realistic details can be added to the model such as the temperature conductivity dependence of any metallic layer and armour permeability. Distributions of magnetic field, current density, resistive losses and temperature inside UGC different layers are calculated at different operating conditions. The exponential increase in conductor temperature with increasing the conductor current limits the single-phasing operation of such cables. Therefore, they must be derated, otherwise their lifetime will be reduced exponentially. Finally, the effect of current harmonics on the temperature distribution inside the insulation material and hence its lifetime is calculated using MATLAB. It is found that higher steady-state conductor temperatures are expected for cables with larger conductor cross-sectional areas, using aluminium core rather than copper, or using 6-pulse rectifiers rather than a higher pulse types.
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